Conveyors of the type in which linkages of the general type of this invention are disclosed in U.S. Pat. No. 3,216,553. The linkages disclosed in this patent were formed by forging or maching the balls and rod from one piece of steel or joining separate balls to a rod by welding, brazing or swedging the rod ends into the balls, or by powdered metal forming as disclosed in U.S. Pat. No. 3,300,303.
The integral machining was found to be very expensive, the forging created linkages which were very inaccurate in pitch and also increased the rod diameter a large amount which decreased the volumetric capacity of the conveyor an undesirable amount, the swedging had to be done before the rods were heat treated because the hardened metal could not be swedged after heat treating without cracking.
In addition, all of these forming procedures did not produce linkages having the maximum tensile strength specified for the type of steel used even after heat treating as required by the steel manufacturer.
After much experimenting, testing and study it was found that it was impossible to heat treat a finished linkage in which where the balls and linkage and been assembled before heat treating because the relatively large mass of a ball around the section of a rod embedded therein shielded the embedded rod section from the effect of the heat treating, particularly in the quenching operation, which caused the rod to always break in the embedded area before the specified tensil strength for the rod had been reached, sometimes as much as by fifty percent.
It was, of course, not possible to weld or braze the balls and rod together after heat treating because the high heat at once destroyed the strength gained by the heat treating.
It was then attempted to achieve the desired tensile strength for the linkages by threading each end of a rod, drilling a central hole in each ball and threading the ball holes to match the rod threads, heat treating the balls and rods separately and attaching the balls and rods after heat treating.
This also failed because the rods always broke in the threaded areas well before the desired tensile strength was reached.